1. Field of the Invention
The present invention relates to an illumination device. More particularly, the present invention relates to an illumination device having a light guide to provide uniform illumination to a target as used in an image reading apparatus such as copying apparatus, facsimile apparatus, scanner and electronic blackboard.
2. Related Background Art
A variety of image reading apparatus having various types of illumination devices have been proposed for image input and conversion of an original document into image signals. For example, U.S. Pat. Nos. 5,808,295 and 5,499,112 disclosed a reading apparatus of an information processing system such as a scanner as shown in FIGS. 1A to 1C, in which Light Emitting Diodes (LEDs) are used to light up the front end of a long, thin light guide that creates a narrow strip of light on a paper target scanned by a linear imaging system.
Referring to FIGS. 1A to 1C, a preferred embodiment of the prior art illumination device is discussed. Numeral 100 denotes the read position of an original, numeral 10 denotes a light transmissive sensor substrate on which a plurality of photo-electric conversion elements formed by using a thin film semiconductor layer are arranged in one dimension. The light transmissive sensor substrate 10 is packaged on a light transmissive packaging substrate 15 by bonding and is electrically connected with a drive circuit. Numeral 30 denotes an illumination means (light source) which comprises LED light sources. Numeral 3 denotes a light transmissive member such as Acrylic, having a 5-sided cross-section as shown in FIG. 1B, the tilted side, which is a 45-degree planar surface that intersects the left side and top side, is named as the illuminating face. The topside is also called top face, and the left side, right side, top face and bottom side would make a rectangle without the intersection of the illuminating face. Numeral 4 denotes the illuminating face. Numeral 9 denotes an incident plane through which a light beam emitted from the illumination means 30 is applied to the illuminating face 4 and top face, and numeral 5 denotes a scatter and reflection area for scattering and reflecting the light beam. The scatter and reflection area 5 is formed by applying light diffusion reflective paint. Numeral 20 denotes a white plastic holder that surrounds all sides of the light guide except incident plane 9, top face and illuminating face 4.
A light beam L emitted from the LEDs in light source 30 enters the light transmissive member 3 from the incident plane 9 of the light transmissive member 3 and repeatedly reflected via the process of total internal reflection ("TIR") at the inner surface of the light transmissive member 3 and propagates therein, until finally reaches the opposite plane to the incident plane 9, where it is again reflected and propagates in the light transmissive member 3. While propagating down the length of the light transmissive member 3, some light reaches the scatter and reflection area 5 where it is diffused and a portion L1 as shown in FIG. 1A and FIG. 1C, is emitted out of the illuminating face 4 located opposing area 5 with an 45-degree angle and it passes through the light transmissive packaging substrate 15 and the illumination window in the light transmissive sensor substrate 10 and irradiates the document sheet located at the read position of an original 100. Another portion L2 as shown in FIG. 1C, of the diffused light beam is directed to the top face and exits. Still another portion L3, as shown in FIG. 1C of the diffused light beam, is directed to the exit plane obliquely so that it is totally reflected as shown as L4 in FIG. 1C of the reflected light beam to exit the illuminating face 4, and another portion L5 as shown in FIG. 1C, of the reflected light beam is again directed to the top face and exits, and some portion of L3 may pass through light transmissive member 3 and hit white plastic holder and bounce back into light transmissive member 3 after suffering an approximately 10% loss, then propagates along with the rest of the reflected light beam in the light transmissive member 3, it repeats the propagation and finally reaches the incident plane 9 where it is emitted out.
The light beam which irradiates the document sheet 100 is reflected by the document sheet 100 and directed to the photo-electric conversion elements on the light transmissive sensor substrate 10 where it is photoelectrically converted to produce an image read signal which is outputed to an external device.
In the example described above, the device may be an efficient extractor of light, but it has some significant problems in regards to its ability to couple the light efficiently with the imaging system. Since the imaging system only receives an extremely narrow line on the scanned paper at any given time came out of the illuminating face 4, the best illumination device should be provided with an illuminating face capable of focusing a scattered light onto the target paper to form a very narrow line. The device as disclosed apparently lacks such ability because the illuminating face of the aforementioned light guide is flat.
The aforementioned devices are further disadvantaged with several more notable problems. First, a large percentage of light leaks out of the top of the light guide as indicated as L2 and L5 in FIG. 1C. Second, the paint stripe surface 5 is not perpendicular to the aim direction of the illuminating face, reducing the amount of directly scattered light out of the light guide towards the center of the target. Thirdly, a white plastic holder that covers substantially the entire surfaces of the light guide except for the top illumination surface contributes further to the loss of light because of absorption of the light by the plastics. As a result of these arrangements, the device as disclosed relies significantly on indirect scatter, which is a less efficient way to illuminate the paper target and widens the illumination pattern.
Referring to FIGS. 2A to 2C, there is shown a perspective view of an additional embodiment of the prior art illumination system. In particular, in FIGS. 2A to 2C, numeral 30 denotes a light source which comprises LED chips 81G and 81R which are light emitting elements having different light emission wavelength ranges. The read position of the document sheet 100, the position of an illumination window and an optical axis along the array direction of the scatter and reflection area 5 of the light transmissive member 3 are set such that they are in a normal plane passing through the read position of the document sheet 100. The light beam L emitted from the LED chips 81G and 81R is scattered and reflected by the scatter and reflection area 5 and a portion L1, of the light beam goes out of the light transmissive member 3, passes through the illumination window to illuminate the original 100. Another portion L2, of the scattered and reflected light beam further propagates through the light transmissive member 3.
In this example, centers of the LED chips 81G and 81R of the respective light emission wavelength ranges of the light source 30 are deviated from a normal line passing through the center of the scatter and reflection area 5, By arranging the light source in this manner, better illumination uniformity along the light guide is achieved, and the high color discrimination ability and multi-tone output image are attained without providing an illumination compensation circuit is produced. In this example, the paint stripe, the scatter and reflection area 5 in FIGS. 2A to 2C, is aligned perpendicular to the aim direction and consequently improved illumination efficiency over the previous example. But the rectangular cross-section of the light guide, which determines the top face of the light guide through which portion L1 of the reflected and scattered light beam travel to the target, is still flat, thus still lacking the ability to focus the light scattered from the paint stripe onto the target.
In addition, while the device as disclosed achieves better illumination uniformity along the light guide by placing the centers of the LED chips 81G and 81R deviated from the normal line, it nevertheless suffers substantial drawbacks as, by its design, it imposes a significant limitation as to the physical location as well as the number of the LED chips permissible on the light guide without compromising illumination uniformity. According to this design, the placement of a third LED chip such as the Blue chip would be difficult, if is not impossible, without compromising its illumination uniformity.
There is therefore an apparent need for an illumination device that has an illumination face capable of forming a focused illumination on the target paper to produce a very narrow line and reduces undesirable light loss from the non-illuminating faces of the light guide, increasing its ability to couple the light efficiently with the imaging system.
There is a further need for an illumination device that can provide greater flexibility as to the placement of LED chip and permits more LED chips without compromising its ability to achieve illumination uniformity along the light guide.
In particular, there is a need for an illumination device to achieve better illumination uniformity along the light guide without the necessity of deviating a center of the LED chips of the respective light emission wavelength ranges of the light source from a normal line passing through the center of the scatter and reflection area.
Accordingly, it is an object of the present invention to provide an illumination device which has an improved light extraction efficiency through better light guide geometry, providing a highly uniform illumination on a target and does not suffer any aforementioned setbacks.
It is another object of the present invention to provide an illumination device to achieve better illumination uniformity along the light guide where LED placement does not have to deviate from the normal line passing through the center of the scatter and reflection area.